Automatic transmissions have been used for some time in different types of motor vehicles. They have a plurality of fixed transmission ratios which can be selected by means of the driver's input and/or by a transmission control device located in the flow of torque between a drive engine and the driven wheels of a vehicle. Between the drive engine of the vehicle and the automatic transmission there is a clutch or driving clutch which allows engagement of a driving gear with the drive engine running and the vehicle stopped. This clutch is often also automatically actuated and thus allows fully automatic or semi-automatic shifting processes.
Automatic transmissions can be designed as synchronized transmissions. In this design, certain transmission elements ensure that, when engaging a gear, the involved, cooperating, form-locking elements of the transmission will move without any, or only a little, desired speed difference. Synchronization is ensured in this design by means of friction couplings within the transmission.
Because of a plurality of different friction pairings for different transmission steps of the transmission placement of a synchronization device in the transmission is associated with fundamental disadvantages related to the costs involved with the additional elements, the needed additional installation space, the greater weight due to the additional elements, and not lastly with regard to the possible maintenance-free life of the transmission. Since friction clutches are subject to significant wear, they have to be replaced after a particular service life, at least in vehicles with high operating power or frequent shifting, and this results in significant expense, due their location in the transmission, and loss of the vehicle during its repair in the workshop.
In particular, with vehicle types which typically have commercial uses and high annual mileage and/or a particularly large number of shift processes, unsynchronized transmissions offer certain advantages. These transmissions are usually designed as “claw” transmissions in which the transmission stage located in the flow of torque between the drive engine and driven wheels is specified by a movement of a form-locked element. Usually in this type of transmission, the gear wheels are secured to a shaft in a non-rotating manner, whereas gear wheels engaging with them are at least seated on and axially fixed to a different shaft as so-called loose wheels, but can rotate radially on the shaft as loose wheels. These loose wheels can be fixed in position on their shaft by means of selector fingers or pawls, so that a flow of torque is possible from a fixed wheel arranged on a first shaft, to a loose wheel secured to a second shaft by a selector pawl.
Shifting of the selector pawls is only possible with reasonable strain on the component and with little or no generation of noise when the loose wheel, secured to its shaft, and the associated shaft in general have roughly the same rotational speed. Since the rotational speed of the transmission output shaft is determined by the cruising speed of the vehicle and by the generally fixed transmission ratios between the driven wheels and the transmission input shaft, the rotational speed of the transmission drive shaft must be adjusted for a gear shift of an unsynchronized transmission in a moving vehicle, so that it at least roughly corresponds to the rotational speed which results after engagement of the claw coupling.
In this regard, the drive engine can be controlled primarily in a simple manner with a transmission in the neutral position and the clutch disengaged, or the engine rotational speed can be controlled so that the drive engine can be set to a higher or lower rotational speed than that needed for synchronization of the transmission, according to the desired acceleration or deceleration of the transmission input shaft. Now if the clutch is slowly engaged, the drive shaft of the transmission will be accelerated or decelerated accordingly. As soon as the deviation from the desired synchronous rotational speed is sufficiently small, the form-locking elements engage, in order to hold the loose wheels, and the desired gear is selected.
This method has the advantage that it will operate problem-free, due to the large potential slippage in the clutch, even with very large differences in rotational speed between the output rotational speed of the drive engine and the actual or desired rotational speed of the transmission drive shaft.
In the case of frequent shifting and with large-weight vehicles, however, the resultant wear on the clutch is significant and, in turn, results in expenses for replacement of the friction linings and also in vehicle down times resulting from the time spent in the workshop. Furthermore, in addition to the friction linings of the clutch the associated actuators and release bearing are placed under stress and subject to wear.
With this as background, there are already some designs for shifting an unsynchronized transmission during vehicle travel with the clutch engaged. In this case it is necessary—at the rotational speed of the transmission output shaft defined by the vehicle's cruising speed—to relatively accurately adjust the rotational speed of the transmission drive shaft and/or of the output shaft of the drive engine connected to and rotating with the output drive shaft via the engaged clutch, to the rotational speed necessary for the synchronization.
Provided this synchronization occurs by means of engine rotational speed within a reasonable time, this requires comparatively complicated and precise engine control. The adjusted rotational speed in this case is initially dependent on the power applied by the drive engine to the pistons, and this power is, in turn, dependent on such numerous factors as the amount of fuel injected, the fuel-air ratio, the ignition angle and also on individually different engine properties within an engine series, and on other factors as well.
For a determination of the engine power that is available for propulsion of the input shaft of the transmission, the engine power applied to the pistons has to be reduced, for example, by internal friction losses, which in turn are dependent not only on the particular amount of wear on the components of the drive engine and the transmission, but also change in the short term, for example, with the temperature and viscosity of the transmission- and engine oil.
Furthermore, it must be taken into account that adjustment of the synchronization speed takes place with the gear disengaged, that is, in the neutral position of the transmission. Owing to the very steep progression of a power-rotational speed curve in this operating state of the drive engine, even small changes in input power or in drag torques counteracting the drive power result in considerable differences in rotational speed, so that, for example, a change in the power consumption of a generator or of an air conditioning system can have short-term, tangible effects on the progression of adjusting the rotational speed. This control range of the drive engine is nonetheless satisfactorily mastered by an engine control in many cases.
Furthermore, adjusting a synchronous rotational speed involves a highly dynamic process in which the mass inertia of the masses to be accelerated or decelerated likewise plays a significant role. Whereas the masses to be accelerated or decelerated are known rather accurately and in general change very slowly, for example, with increasing wear on the clutch linings, it must further be taken into account that the synchronization rotational speed during the shifting process can likewise change on the transmission input side. As soon as the traction power of the engine can no longer be transferred to the driven wheels because the gear, previously engaged in the transmission, is disengaged and the transmission is thus in the neutral position, the speed of the vehicle changes according to the applied forces which depends on the inclination of the surface on which the vehicle is located, the vehicle weight, roll resistance caused by vehicle components and road surface roughness, air resistance, and the speed and direction of incident air as well as other factors. These factors are of course not within the ability of the engine control to ameliorate and must therefore be detected separately and passed to the engine control as a synchronous rotational speed needed at present or in the future.
For the decision whether a gear shift should be performed without disengaging the clutch and for sufficiently accurately setting a synchronous rotational speed, the interplay of these and other factors has to ultimately be estimated. It must be taken into account that the advantages attainable by a shifting process without disengaging of the clutch are largely based on a reduction in the wear.
Of course, it is sufficient in the simplest case to determine the desired synchronous rotational speed of the transmission input shaft by the rather easily measured rotational speed of the transmission output shaft and the translation known by the target gear, and also to influence the output shaft of the drive engine in the desired direction by means of a control loop. But a procedure of this kind often leads to disproportionately long shift times or an undesirable, imprecisely adjusted synchronous rotational speed during highly fluctuating vehicle operating conditions.
Finally, it should be noted that there are shifting processes which cannot be, or, in any event, cannot advantageously be implemented without disengaging the clutch. Among these are not only start up processes, but also shift processes which would require, for example, a rotational speed of the drive engine below the normal idle rotational speed or even below the possible idle rotational speed. A shifting process of this kind can be desirable, for instance, when a vehicle is moving downhill and the driver wants only a very small effect of the engine brake or in anticipation of an acceleration of the vehicle on steep inclines, when the driver wants to select a gear that will briefly cause engine operation at or below the lower rotational speed limit, but will be at an appropriate driving gear within a few seconds based on vehicle acceleration.
From DE 102 49 951 A1 a method to control a drive train is already known in which a gear shift is made possible by taking into account a plurality of different factors, but without disengaging the clutch. In this regard, for each gear shift a decision is made, whether the gear shift is to be performed with clutch engaged or disengaged. Provided that an analysis shows a gear shift is possible with the clutch engaged, the gear shift will also be carried out with the clutch engaged.
It is proposed in this regard to select and/or analyze the mode of gear shift based on a plurality of vehicle parameters and operating characteristics. It should be stressed that the danger that the selection of the mode of gear shift will result in a wrong result is particularly low with this method. On the one hand, this is due to an extremely complicated process sequence, both with respect to the needed computational power of an electronic transmission control unit, and also due to the needed sensors, and, on the other hand, because a gear shift with an engaged clutch can only be carried out when the determination method will predict a smooth shift with a high degree of reliability.
For example, according to one basic variant of the method disclosed in DE 102 49 951 A1, shifting with the clutch engaged is not used when one of the involved components or its sensors happens to be malfunctioning. Due to the mentioned large number of factors and components to be taken into account, this means in practice a significant reduction in the average availability of the system. Furthermore, after the initial start of the vehicle and after a restart, very restrictive rules are set for allowing a shift process with the clutch engaged, in order thus to compensate for the uncertainty regarding as yet insufficiently known current and critical influence parameters.
The security against a wrongly permitted shifting process with an engaged clutch is greatly increased, when even small uncertainties are detected with regard to the feasibility, the system is switched to a shifting mode with disengaged clutch. The advantages of shifting with the clutch engaged can thus not be realized in many cases in which they would be useful in practice, for reasons considered in this known procedure. It is thereby important that the decision about the mode of gear shifting (with an engaged clutch or a disengaged clutch) is always made before initiation of the shift procedure.
Accordingly, the envisioned procedure is not able, at least in some cases, to guarantee the claimed certainty of the shift mode decision. To be considered thereby is the daily major change of loading in commercial vehicles and thus the overall vehicle weight, such that after restarting the drive motor a shift with an engaged clutch can be omitted until the basic influence factors, like, for example, the overall vehicle weight, can be determined to a sufficiently accurate extent or can be estimated. Now, however, shutting off the drive motor is not urgently necessary for a basic change of load and is more the exception than the rule with, for example, construction site vehicles like dump trucks and cement mixers, certain tanker vehicles, but also with passenger vehicles, like school and tour buses.
In order to eliminate at least in part the described disadvantages shown in DE 102 49 951 A1, an improvement of this process is, as proposed in the invention, to less stringently formulate the requirements for enabling a shift procedure with an engaged clutch and thereby, with a decision made for a gear change with an engaged clutch, to also consider the partial step of disengaging the previously engaged gear to check the appropriateness of the decision.
In fact it is assumed that, if a wrong decision is made about the mode of gear shifting, the transmission cannot be placed into an essentially torque-free state, so that disengagement of the engaged gear is possible at low shifting force within a default period of time.
Thus in the event of a decision made for a gear shift with an engaged clutch, a time limitation is introduced for disengagement of the engaged gear, and, if this time is exceeded, it is assumed that the necessary prerequisites for the process of engagement of the target gear with an engaged clutch have not been satisfied. In this case, the shifting process will be terminated and will begin anew in a shifting mode with a disengaged clutch, or the system will switch to this mode.
Thus the additional advantage of this improvement to the known method is that a shifting process already initiated can still be terminated at a relatively late point in time. It should be recalled, however, that the requirements of accuracy of the engine control for the disengagement of a gear are much looser than for the successful engagement of a gear. Successful disengagement of a gear thus cannot be judged as a dependable sign that the subsequent engagement of a gear with an engaged clutch will be successful or desirable.
Furthermore, the requirements for a shifting process with an engaged clutch differ from gear to gear, and, for example, as a function of the driving speed and of the loading and other operating parameters of the motor vehicle and on its environment. This is to be taken into account according to DE 102 49 951 A1, in that the threshold time until termination of the attempted gear shift with an engaged clutch is varied depending on parameters of this type, which in turn increases the complexity of the method and thus either increases its susceptibility to wrong decisions or considerably increases the needed safety margin to ensure a still advantageously implemented shifting process with an engaged clutch.